With improvements in both braze and tube extrusion technology, it has been possible to improve automotive air conditioning system condensers in several obvious ways. Improved brazing materials and fluxes have allowed manufacturers to consistently achieve leak free tube end to header slot braze joints, in turn allowing for a shift from long, serpentine tube designs to shorter, multi tube flow designs. These are sometimes referred to as "parallel flow" designs, because of the fact that the flow tubes are parallel to each other, but this is somewhat inaccurate, since the flow passes of a serpentine tube are also parallel to each other. "Cross flow" is a more accurate description of the two significant flows involved in the condenser, the refrigerant flowing from side to side (or up and down) through the refrigerant flow tubes, from one header to the other, and the forced outside air running perpendicular thereto, "crossed" with the refrigerant flow. With multiple, shorter tubes, smaller end to end refrigerant pressure drops create the potential for smaller flow passages in thinner tubes. At the same time, improvements in extrusion technology have allowed thinner flow tubes to be integrally extruded, instead of fabricated from thinner pieces, which has been the desired design direction of the industry for at least three decades. Thinner extruded tubes, in turn, have smaller free flow areas, with a higher surface area to internal volume ratio, both of which obviously improve thermal performance.
Since condenser tubes have become thinner and consequently more thermally efficient, it has been possible to make them narrower, as measured in the direction of air flow, giving cores of smaller depth, although the tubes are still far wider in cross section than they are thick. With narrower tubes, cylindrical headers are feasible, since narrower tube ends allow for smaller diameter cylindrical headers, with less volume and weight. Cylindrical headers are also inherently better pressure vessels. Cylindrical headers can be two piece structures, with separate, half cylinder tank bodies and slotted header plates brazed lengthwise thereto, or they can be one piece cylindrical tanks, with one side regularly slotted to receive the equally spaced tube ends. In older, rectangular header designs, the slotted header plate portion of the tank is curved slightly, but not as steeply curved as with a cylindrical tank, in which the cross section of the header plate is basically a semi circle. The braze seam interface between the flat tube end and the header plate is also, therefore, basically a semi circle. In operation, the condenser is subjected to thermal cycling forces, and to bending stresses in the flow tube which are concentrated, in cantilever fashion, at the interface between tube end and curved header. With a flatter header plate, the bending stress is more evenly distributed across the width of the tube, but with a highly curved header plate, it tends to be more highly concentrated in an area at the peak of the curve, centrally of the tube end. Such concentration of stresses can lead to stress fracture, with time, especially with the thinner and less stress resistant flow tubes that can now be successfully extruded.
The cross sectional shape of extruded condenser tubes has been driven by the obvious expedient of maximizing free flow area of the refrigerant. Consequently, by far the most common cross sectional tube configuration has been a simple series of evenly spaced, nearly square and sharp cornered flow passages, separated by regularly spaced internal webs of constant thickness. When a thin tube end is subjected to concentrated stresses, as described above, the sharp internal corners in the square flow passages can act as stress risers that exacerbate the onset of stress fracturing. Round or curved edged passages are not unknown, but are less common, since they inherently pack less refrigerant free flow area and volume into a given tube cross sectional area, for the same reason that round cans occupy less of a shelf's space than do square boxes. Even round flow passages would not solve the cracking problem alone, since there is simply not enough strength in the tube end's area of maximum stress concentration.
There are rare exceptions to the rule of evenly spaced, constant size flow passages and webs in flow tubes, but these tube designs are not directed toward the resistance of stress cracking at the tube end. One example can be seen in published UK Patent Application GB 2 133 525, a 1984 publication which shows an extruded tube cross section with square cornered flow passages of progressively decreasing width, moving in the direction of air flow across the tube. This is directed toward increased corrosion resistance, putting thicker outer wall sections where the corrosion is worse. Even there, the internal web thicknesses are fairly regular. A coassigned patent U.S. Pat. No. 5,186,246 discloses a combined radiator and condenser with an integrally extruded double flow tube which, if a cross section were taken not through the tube end, would have the appearance of a single tube with unevenly spaced flow passages of different size. This appearance flows from the fact that the tube is two sided, or two tubes joined along their inner edges, in effect. The integral double tube has a series of smaller condenser flow passages on one side, and a single large radiator passage on the other side, separated by a wider central area having no flow passages. The central area joining the two sides of the single tube is undesirable in terms of thermal efficiency, however, since it promotes cross heat flow between the condenser and the radiator, and is simply an inevitable result of extruding an integral, two sided tube. Alternate embodiments provide two totally separate tubes. Moreover, the central area in the integral double tube embodiment does nothing to resist stress cracking at the tube end, since it is has to be notched and cut entirely away in order to allow the tube end to be inserted into the dual header tank without interference.
In short, the standard for an extruded condenser tube is a tube cross section with regularly spaced, uniform sized, square or rectangular flow passages separated by regularly spaced, uniformly thin internal webs. Thick internal webs would remove too much refrigerant free flow area, and are therefore extruded no thicker than the simple requirement for tube burst resistance required.